Liquid crystal displays (LCDs) are commonly used in battery operated equipment, such as cell phones, personal digital assistants (PDAs), and laptop computers, and are replacing bulky CRTs as television screens and computer monitors. Presently, drawbacks of such LCDs include limited brightness, low efficiency, and limited viewing angle. LCDs can be monochrome or color and can be transmissive or reflective. The present invention deals with a color, transmissive LCD that uses backlighting.
FIG. 1 is a cross-sectional view of a small portion of a prior art color, transmissive LCD. There are other types of color, transmissive LCD structures. The structure of FIG. 1 will be used to identify certain disadvantages of prior art LCDs that are avoided by the present invention.
In FIG. 1, an LCD 10 includes a white light source 12 to provide backlighting for the upper LCD layers. A common source for white light is a fluorescent bulb. Another white light source is a combination of red, green, and blue light emitting diodes (LEDs) whose combined light forms white light. Other white light sources are known. These white light sources must provide homogeneous light to the back surface of the display.
A popular technique for providing such a homogeneous white light is to optically couple the fluorescent bulb or LEDs to a light guide, such as by optically coupling the light source to one or more edges of a sheet of clear plastic. The sheet has deformities that bend the light approximately normal to the top surface of the sheet so that light is emitted from the surface. Examples of such deformities include ridges in the bottom surface, reflective particles embedded into the plastic sheet, or a roughening of the top or bottom surface of the sheet. The deformities cause a quasi-uniform plane of light to be emitted out the front surface of the light guide. A non-specular reflector may be placed behind the back surface of the light guide to improve brightness and uniformity.
It is also common to not use any light guide but position a light source behind the display with appropriate diffusers to uniformly distribute the light across the display.
A polarizing filter 14 linearly polarizes the white light. The polarized white light is then transmitted to a transparent thin film transistor (TFT) array 16 having one transistor for each pixel. TFT arrays are extremely well known and need not be further described.
The light output from the TFT array 16 is then filtered by an RGB pixel filter 18. The RGB filter 18 can be located at other positions in the stack. The RGB pixel filter 18 may be comprised of a red filter layer, a green filter layer, and a blue filter layer. The layers may be deposited as thin films. As an example, the red filter contains an array of red light filter areas defining the red pixel areas of the display. The remaining portions of the red filter are clear to allow other light to pass. Accordingly, the RGB pixel filter 18 provides a filter for each R, G, and B pixel in the display.
Above the RGB pixel filter 18 is a liquid crystal layer 20, and above liquid crystal layer 20 is a transparent conductive layer 22 connected to ground. The absence of an electrical field across a pixel area of the liquid crystal layer 20 causes light passing through that pixel area to have its polarization rotated orthogonal to the incoming polarization. An electrical field across a pixel area of the liquid crystal layer 20 causes the liquid crystals to align and not affect the polarization of light. Selectively energizing the transistors controls the local electric fields across the liquid crystal layer 20.
A polarizing filter 24 only passes polarized light orthogonal to the light output from the polarizing filter 14. Therefore, the polarizing filter 24 only passes light that has been polarized by a non-energized pixel area in the liquid crystal layer 20 and absorbs all light that passes through the energized portions of the liquid crystal layer 20. The magnitudes of the electric fields across the liquid crystal layer 20 control the brightness of the individual R, G, and B components to create any color. In this manner, any color image may be presented to the viewer by selectively energizing the various transistors in the TFT array 16.
Other types of LCDs substitute a passive conductor grid for the TFT array 16, where energizing a particular row conductor and column conductor energizes a pixel area of the liquid crystal layer at the cross-point. Other types of LCDs do not use polarizing filters.
The RGB pixel filter 18 inherently filters off two-thirds of all light reaching it, since each filter only allows one of the three primary colors to pass. This is a significant factor in the generally poor efficiency of the prior art LCDs. The overall transmissivity of the LCD layers above the white light source 12 is on the order of 4-10%.
What is needed is a technique for increasing the brightness of an LCD output without requiring additional energy for the light source.
Another problem with a backlight outputting R, G, and B components is that the liquid crystal layer needs to have consistent properties for all the visible wavelengths. Optimizing the shutter function of the liquid crystal layer for each color is not possible, and a non-optimum compromise for each color must be made. Also, angular viewability and changes of color with viewing angle have always been issues with conventional LCDs. In order to increase the viewing angle, both the liquid crystal layer and the backlight have to be tailored to emit light through a broad range of angles. However, the wider the output light angle, the poorer the modulating efficiency of the liquid crystal layer.
What is also needed is a technique to improve the optical qualities of the liquid crystal layer.
An alternative LCD uses as a backlight a UV source and a phosphor-coated plate that emits a white light when energized by the UV. However, UV can be hazardous, and the LC and plastics deteriorate rapidly when exposed to UV. Suitable UV sources are also relatively expensive.